1. Field of the Invention
The present invention relates to a semiconductor device and more particularly, to a semiconductor device equipped with diodes connected in series such as that having a reference voltage generating diode and a temperature-compensating diode therefor, which is used for a reference voltage generator in measurement systems or the like.
2. Description of the Prior Art
FIG. 1 shows an example of conventional semiconductor devices of this sort. In FIG. 1, a Zener diode ZD is provided on a first main surface of an n-type silicon substrate 11 to generate a reference voltage, and a temperature-compensating diode Di is provided on second main surface of the substrate 11 to compensate the temperature characteristic of the Zener diode ZD. The diodes ZD and Di are connected in series with each other through the substrate 11.
A silicon dioxide film 12a having a window is formed on the first main surface of the substrate 11. A p-type diffusion region 14 is formed in the substrate 11 along the periphery of the window to act as a guard ring for the Zener diode ZD. The circular ringed diffusion region 14 is deposited in the surface area of the substrate 11 adjacent to the first main surface.
A p-type patterned polysilicon film 15 as an anode is formed on the exposed part of the substrate 11 to be in contact with its first main surface through the window of the silicon dioxide film 12a. The n-type silicon substrate 11 and the p-type polysilicon film 12a form a main p-n junction of the Zener diode ZD at their contact area surrounded by the guard ring 14. The fringe of the polysilicon film 15 is deposited on the silicon dioxide film 12a.
A silver (Ag) bump 17a as a first electrode is formed on the polysilicon film or anode 15.
A silicon dioxide film 12b having a window is formed on the second main surface of the substrate 11, which is opposite to the first main surface. A p-type diffusion region 13 as an anode is formed in the substrate 11. The region 13 is deposited in the surface area of the substrate 11 adjacent to the second main surface and in the position opposite to the p-type diffusion region 14. The p-type diffusion region 13 and the n-type substrate 11 form a p-n junction of the temperature-compensating diode Di at their contact area.
A silver bump 17b as a second electrode is formed on the exposed part of the substrate 11 to be in contact with the diffusion region 13 through the window of the silicon dioxide film 12b. The fringe of the silver bump 17b is deposited on the silicon dioxide film 12b.
The substrate 11 acts as cathodes of the diodes ZD and Di, which are coupled together.
The conventional semiconductor device as described above is fabricated through the following process sequence.
First, the silicon dioxide films 12a and 12b are formed on the first and second main surfaces of the n-type silicon substrate 11, and the windows are formed in the films 12a and 12b, respectively, as shown in FIG. 1.
Next, a p-type impurity is selectively diffused through the windows into the substrate 11 to form the p-type diffusion region 14 as the guard ring of the Zener diode ZD and the p-type diffusion region 13 as the anode of the temperature-compensating diode Di, respectively.
Then, the p-type patterned polysilicon film 15 as the anode is formed on the exposed part of the substrate 11 to form the main p-n unction of the Zener diode ZD.
Finally, the silver bump 17a as the first electrode is formed on the p-type polysilicon film 15 and the silver bump 17b as the second electrode is formed on the p-type diffusion region 13. Thus, the conventional semiconductor device shown in FIG. 1 is finished.
With the above-mentioned conventional semiconductor device shown in FIG. 1, the Zener diode ZD and the temperature-compensating diode Di are formed at each side of the substrate 11, in other words, the semiconductor device is of the double-sided structure. Therefore, the Double Heatsink Diode (DHD)-type package has been applied for the device.
There is a problem that the conventional semiconductor device cannot be packaged using the die bonding (or pellet mounting) method and the wire bonding method, which means that the plastic-molded package and the can and ceramic casings do not available for the device. Also, since only the DHD-type package is available, the device cannot be made as a surface-mount device.
There is another problem that the electrical characteristics of the conventional semiconductor device are difficult to be tested in the state of the wafer because of its double-sided structure.
There is still another problem that one of the diodes ZD and Di must be protected in the dicing process because of its double-sided structure, which makes the dicing process complicated.